From Sulfur−Amine Solutions to Metal Sulfide Nanocrystals: Peering into the Oleylamine−Sulfur Black Box

In this Article, we present our findings on the formation of metal sulfide nanocrystals from sulfur−alkylamine solutions. By pulsed field gradient diffusion NMR along with the standard toolbox of 1D and 2D NMR, we determined that sulfur−amine solutions used as a sulfur precursor exist as alkylammonium polysulfides at low temperatures. Upon heating to temperatures used in nanocrystal synthesis, the polysulfide ions react with excess amine to generate H2S, which combines with the metal precursor to form metal sulfide. Four different reaction pathways were found, each of which produced H2S and the byproducts identified in this Article. Thioamides were identified as an intermediate and were shown to exhibit much more rapid kinetics than sulfur−alkylamine solutions at low temperatures in the synthesis of metal sulfide nanocrystals.